1. Field of the Invention
The present invention is concerned with the structure of cylinder heads of internal combustion engines and, more particularly, with the cylinder head structure of four valve internal combustion engines which are endowed with two intake valves and two exhaust valves for each cylinder.
2. Description of Related Art
Generally, automobile internal combustion engine design focusses on the compactness of combustion chambers in order to achieve improvements in combustion (i.e., improved thermal efficiency, improved knock resistance, etc.). In particular, four valve engines, which are endowed with two intake valves and two exhaust valves for each cylinder, are arranged so that each intake valve is located in close proximity to an exhaust valve. Most designs concentrate on making the combustion chamber compact. Ordinarily, since the intake valve is made larger than the exhaust valve, due to a demand for improved charging efficiency, the intake ports and the exhaust ports, which open into the combustion chamber, are, respectively, aligned on straight lines offset from and on opposite sides of a diametric line of the combustion chamber. The intake ports and the exhaust ports are arranged so as to be adjacent to the vertical center line of the combustion chamber. Each intake port is located closer to the center of the combustion chamber than the exhaust port. Such an intake and exhaust port arrangement is known from, for example, Japanese patent application No. 63-273,746, entitled "Combustion Chamber of Engine," filed on Oct. 28, 1988 and published as Japanese Unexamined Patent Publication No. 2-119,617 on May 7, 1990.
In the arrangement described in the publication mentioned above, in which the two exhaust ports are in close proximity to each other, the region of a combustion chamber in which the two exhaust ports open reaches high temperatures easily. This results in knocking and, specifically, in pre-ignition relating knocking. Particularly, in supercharged engines, since one of the factors determining how efficiently an engine provides its power while it operates under supercharging is the occurrence of knocking, the occurrence of such knocking due to the arrangement of exhaust ports is a serious problem. Furthermore, when operating such an engine under supercharging, since an extremely high thermal load occurs in the cylinders, the reliability of parts of the cylinder head must be improved. However, in an engine which has two exhaust ports arranged in close proximity to each other, a portion, or what is called a "valve bridge," between the two exhaust ports of the cylinder head, as well as a portion between each of the exhaust ports and a spark plug, is relatively thin in section. The valve bridge, therefore, cannot be adequately cooled. Consequently, under a high thermal load, the engine has a decreased reliability.